The present invention relates to a piezoelectric transformer element suitably used to, e.g., turn on a cold cathode tube, and a method of housing the same.
A cold cathode tube is generally used for the backlight of the liquid crystal display of a notebook personal computer or the like. This cold cathode tube requires high voltage of about 1 kV to turn it on, and voltage of several hundred V to keep it on. The cold cathode tube can be kept on at voltage much lower than the voltage required to turn it on. The operation characteristics of the cold cathode device are similar to the operation characteristics of a piezoelectric transformer element, so that an inverter having such a piezoelectric transformer element is recently being used as a power supply for the cold cathode tube.
As the piezoelectric transformer element, a Rosen piezoelectric transformer element like the one shown in FIG. 21 is widely used.
FIG. 21 is a perspective view showing a general Rosen piezoelectric transformer element. In FIG. 21, a piezoelectric transformer element 106 has a rectangular shape, its left half in FIG. 21 serves as an input region, and its right half in FIG. 21 serves as an output region. External electrodes 101 and 102, as input electrodes for applying input voltage, are formed on the upper and lower surfaces of the input region. An external electrode 103, as an output electrode for extracting boosted AC voltage, is formed on the end face of the output region. When an AC voltage (input voltage) having a resonance frequency determined by the material and length of the piezoelectric transformer element 106 is applied between the external electrodes 101 and 102, the piezoelectric transformer element 106 mechanically vibrates and resonates in the longitudinal direction. The mechanical vibration generates boosted AC voltage between either one of the external electrodes 101 and 102 as a ground electrode common to the input and output sides, and the external electrode 103. The boosting ratio as the ratio of the output voltage to the input voltage is proportional to the ratio of the distance between the output electrodes and the distance between the input electrodes.
As the piezoelectric transformer element, a single-plate piezoelectric transformer element as the one shown in FIG. 21 has conventionally been used. In an inverter having such a single-plate piezoelectric transformer element, the boosting ratio is about 10 at most. To turn on a cold cathode tube for a notebook personal computer, a boosting winding transformer must be provided on the input stage of the piezoelectric transformer element. Accordingly, to obtain a higher boosting ratio than that of the single-plate piezoelectric transformer element, and to eliminate the winding transformer from the input stage, a piezoelectric transformer element having a structure in which the distance between internal electrodes in the input region of piezoelectric layers is decreased, i.e., the thin piezoelectric layers and the internal electrodes are stacked, is formed.
In this multilayer piezoelectric transformer element, the electrodes in the input region must be electrically connected. As the connection method, for example, Japanese Patent Laid-Open No. 7-302938 discloses the method in which electrodes 305 for electrically connecting internal electrodes 301a and 302a in the input region between external electrodes 301 and 302 are formed at the corners of the side or end faces of a piezoelectric transformer element 306, as shown in FIG. 24. In Japanese Patent No. 8-52553, the present applicant proposed a method of alternately connecting a plurality of internal electrode layers stacked inside the input region of a piezoelectric transformer element by two columnar conductors (to be referred to as interlevel connection conductors hereinafter).
When such a piezoelectric transformer element is mounted on a circuit board, the piezoelectric transformer element is often used in a housing so as not to expose the high-voltage portion. In a conventional multilayer piezoelectric transformer element, lead wires are bonded with solder or conductive adhesive to external electrodes formed on the upper and lower surfaces of the input region, similar to the external electrodes 101 and 102 in FIG. 21. Although the piezoelectric transformer element has the advantage of a small thickness, mounting the piezoelectric transformer element connected to the lead wires requires a margin of at least the diameter of the lead wire between the element and the board, and a space for at least the diameter of the lead wire above the piezoelectric transformer element. As a result, the thickness of the housing increases, and the advantage of the thin piezoelectric transformer element cannot be fully exploited.
The housing must be designed to have a large thickness in consideration of the diameter of the lead wire and the thickness of solder or conductive adhesive for bonding the lead wire. The thickness of the piezoelectric transformer module upon mounting, therefore increases with xe2x80x9cthickness for the bonding spacexe2x80x9d.
To solve this problem, for example, according to Japanese Patent Laid-Open No. 9-116250, input electrodes 201 and 202 are formed on upper and lower surfaces on the primary side of a piezoelectric transformer element 206 and extended to the side surfaces of the element to form lead electrodes 201a and 202a so as to fix the piezoelectric transformer element 206 to a housing 205 by terminals 210A and 210B at node positions of the side surfaces in the longitudinal direction, as shown in FIGS. 22 and 23. Output electrodes 204 are formed on the two side surfaces of the element on the secondary side. This structure can make the housing thin and simplify the mounting process.
However, with recent developments in notebook personal computers or portable information terminals, demands have arisen for smaller-size apparatus. Along with this, the difference in size between the liquid crystal display and the main body is diminishing. Since an inverter for the backlight of the liquid crystal display is generally arranged around the liquid crystal display, the inverter must be narrowed as the outer dimension of the main body of a personal computer or the like is downsized.
By the conventional housing method, however, the housing or module can be made thin, but the housing cannot be narrowed.
It is an object of the present invention to provide a piezoelectric transformer element whose height and width necessary for mounting can be decreased, and a method of housing the same.
To achieve the above object, a piezoelectric transformer element of the present invention is characterized by the following arrangement.
A piezoelectric transformer element with a multilayer structure having an input region which is polarized in a direction of thickness and has input electrodes, and an output region which is polarized in a longitudinal direction and has an output electrode is characterized by comprising a plurality of internal electrodes as the input electrode which are formed between a plurality of piezoelectric layers, first and second external electrodes formed on an end face of the piezoelectric transformer element in the longitudinal direction on the input region side, and an external electrode formed on an end face of the piezoelectric transformer element in the longitudinal direction on the output region side, wherein the plurality of internal electrodes are alternately connected to the first and second external electrodes every other layer, and the external electrodes are electrodes for applying input voltage. With this structure, when the piezoelectric transformer element is mounted on another member, the element can be connected at only its end face in the longitudinal direction, and the height and width (width of widthwise direction of the element) necessary for mounting can be reduced.
The element preferably further comprises third and fourth external electrodes formed on side surfaces parallel to the longitudinal direction of the piezoelectric transformer element on the input region side, and the plurality of internal electrodes are also alternately connected to the third and fourth external electrodes every other layer.
The element preferably further comprises a plurality of columnar conductors formed inside the input region, and the plurality of internal electrodes are alternately connected via the plurality of columnar conductors every other layer.
This arrangement can greatly reduce the possibility of failing to drive the piezoelectric transformer element.
The third and fourth external electrodes or the plurality of columnar conductors are desirably formed at positions corresponding to vibration nodes of the piezoelectric transformer element.
To achieve the above object, a piezoelectric transformer element housing method of the present invention is characterized by the following steps.
More specifically, a piezoelectric transformer element housing method of mounting the piezoelectric transformer element having the above arrangement on a housing is characterized by comprising the steps of forming a plurality of mount terminals on two end faces of the housing in a longitudinal direction, inserting the piezoelectric transformer element in the housing to make the plurality of mount terminals abut against the first and second external electrodes and the output electrode, and fixing the abutted portions by a conductive connection material. By this method, the piezoelectric transformer element can be mounted on the housing capable of connecting the element at only its end face in the longitudinal direction, the height and width (width of widthwise direction of the element) necessary for mounting can be reduced, and the manufacturing process can be simplified.
Alternatively, a piezoelectric transformer element housing method of mounting the piezoelectric transformer element having the above arrangement on a housing is characterized by comprising the steps of forming projections near positions inside the housing where two end faces of the element in a longitudinal direction are located after the piezoelectric transformer element is housed, and supporting the piezoelectric transformer element by the projections, connecting, via lead wires, the first and second external electrodes to a plurality of mount terminals formed on an end face of the housing in the longitudinal direction, and connecting, via a lead wire, the output electrode to a mount terminal formed on the other end face of the housing in the longitudinal direction. By this method, the piezoelectric transformer element can be mounted on the housing capable of connecting the element at only its end face in the longitudinal direction, and the height and width (width of widthwise direction of the element) necessary for mounting can be reduced.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.